This is common for homes built from the 1960’s-present day. In this type of construction the outer nailing flange of a window is used. When a home is built, it is framed, there is a sub-sheathing that is applied (a type of vapor barrier), and then the siding is installed. The existing nailing fin of the window is either applied directly to the stud or the sub-sheathing. Then the siding or trim covers the flange.
We cut the siding or trim back to access the nailing fin in order to remove the existing window. We incorporate our flashing, metal z-flashing, caulking, and trim in a way that ensures you that your windows will be performing great well into the future.
Some other contractors will cut the nailing fin off. In the industry this is referred to as a “glue and screw”. This method is not recommended by the manufacturer and relies on the exterior caulking to prevent water infiltration. Would you want a single bead of caulking to be the only barrier from water entering the wall cavity of your home? Here is where you see the “right way vs wrong way”.